Resistance is measured in ohms and resistivity depends on the material of the wire, so this Class 12 current-electricity sheet begins with the units and the cause of resistance. The key answers 1 Ω for a later resistor and P/Q = R/S for a Wheatstone bridge condition.
Key Takeaways
The SI unit of resistance is the ohm (Q1).
Resistivity depends on the material of the wire (Q2).
The key records 1 Ω and the Wheatstone condition P/Q = R/S.
Section A: Multiple Choice Questions (1 Mark Each)
Choose the correct option for each question.
1.The SI unit of electrical resistance is the:
(a) ohm(b) ampere(c) volt(d) coulomb
2.The resistivity of a wire depends on the:
(a) length of the wire(b) material of the wire(c) diameter of the wire(d) current through the wire
3.Three resistors of 2 Ω, 3 Ω and 6 Ω are connected in parallel. The equivalent resistance is:
(a) 1 Ω(b) 11 Ω(c) 6 Ω(d) 3 Ω
4.The Wheatstone bridge is balanced when:
(a) P = Q = R = S(b) P/Q = R/S(c) P × Q = R × S(d) P + Q = R + S
5.The drift velocity of electrons in a current-carrying conductor is typically of the order of:
(a) mm/s(b) m/s(c) km/s(d) cm/s
6.The internal resistance of an ideal battery is:
(a) zero(b) one ohm(c) infinite(d) equal to the external resistance
Section B: Short Answer Type Questions (2 Marks Each)
Show all steps clearly.
7.State Ohm's law and the conditions under which it is valid.
8.Define resistivity of a material and give its SI unit.
9.State Kirchhoff's junction rule and Kirchhoff's loop rule.
10.Describe the Wheatstone bridge and write the condition for its balance.
11.Define drift velocity and write the relation between current and drift velocity.
12.Distinguish between the emf and the terminal potential difference of a cell.
13.Write the expressions for the equivalent resistance of resistors R₁ and R₂ connected in series and in parallel.
14.Define the temperature coefficient of resistance.
Section C: Numericals & Word Problems (3 Marks Each)
Apply the concepts to solve the problems. Show all working.
15.A wire of resistance 10 Ω is stretched to twice its original length so that its volume remains constant. Using R ∝ L²/A and R ∝ L² when volume is fixed, find its new resistance.
16.A battery of emf 12 V and internal resistance 1 Ω is connected to a 5 Ω resistor. Using I = E/(R + r), find the current in the circuit and the terminal voltage of the battery.
17.Resistors of 2 Ω, 3 Ω and 6 Ω are connected in parallel across a 12 V battery. Using 1/Rp = 1/R₁ + 1/R₂ + 1/R₃ and I = V/R, find the total current drawn.
18.A current of 2 A flows through a conductor. Using I = ne, where e = 1.6 × 10⁻¹⁹ C, find the number of electrons crossing a cross-section per second.
19.A wire of length 1 m and cross-sectional area 2 × 10⁻⁶ m² has a resistivity of 2 × 10⁻⁸ Ω·m. Using R = ρL/A, find its resistance.
20.A heater draws a current of 5 A from a 220 V supply. Using P = VI and E = Pt, find the power of the heater and the energy consumed in 2 hours in kWh.
Answer Key
1. a) ohm
2. b) material of the wire
3. a) 1 Ω
4. b) P/Q = R/S
5. a) mm/s
6. a) zero
7. Refer to solution guide.
8. Refer to solution guide.
9. Refer to solution guide.
10. Refer to solution guide.
11. Refer to solution guide.
12. Refer to solution guide.
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14. Refer to solution guide.
15. Refer to solution guide.
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20. Refer to solution guide.
End of Worksheet
People Also Ask
What is the unit of electrical resistance?
The ohm, option (a) in this worksheet (Q1).
What does the resistivity of a wire depend on?
The material of the wire, option (b) in this worksheet (Q2).
What is the Wheatstone bridge balance condition?
P/Q = R/S, from the answer key on this page.
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